Adsorptive Separation of CO2 by a Hydrophobic Carborane-Based Metal–Organic Framework under Humid Conditions

We report that the carborane-based metal–organic framework (MOF) mCB-MOF-1 can achieve high adsorptive selectivity for CO2:N2 mixtures. This hydrophobic MOF presenting open metal sites shows high CO2 adsorption capacity and remarkable selectivity values that are maintained even under extremely humid conditions. The comparison of mCB-MOF-1' with MOF-74(Ni) demonstrates the superior performance of the former under challenging moisture operation conditions.


■ INTRODUCTION
The global greenhouse effect, which is mostly caused by the emission of CO 2 into the atmosphere, has attracted more and more attention. 1 Apart from natural processes, the consumption of carbon-based fossil fuels from power plants is mainly responsible for generating this anthropogenic CO 2 emission. 2,3 Therefore, carbon capture and storage (CCS) from the postcombustion flue gas is a wise method to reduce CO 2 environmental impact. So far, a number of technologies and materials have been developed for CO 2 capture and separation, such as aqueous ammonia and amine-functionalized solid adsorption, 4−6 membrane separation, 7,8 cryogenics distillation, 9−11 among others. Compared with traditional techniques, adsorption-based methods using porous materials to capture or separate CO 2 with less energy consumption and cost shows great advantages among these technologies. Activated carbon, zeolites, and carbon molecular sieves have been extensively studied as adsorbents for CO 2 separation. 12−14 These materials present several drawbacks such as difficult regeneration procedures and, quite importantly, poor tunability with the procedure conditions. Thus, developing economical and highly regenerative materials to efficiently capture and separate CO 2 from flue gas is highly desirable.
Metal−organic frameworks (MOFs) exhibit outstanding separation performances toward diverse binary gas mixtures due to their large surface areas, tunable pore size and pore surface, and existing open metal sites. 15−18 Many MOFs have been reported to efficiently separate CO 2 :N 2 mixtures under dry conditions; however, their performance cannot be often maintained under humid conditions. 19,20 The reason for such a decrease in performance can be twofold. On one hand, there are MOFs that are unstable upon exposure to water and thus cannot be used in gas separation under humid conditions. This is the case, for example, of two of the most well-known adsorbents, HKUST-1 ([Cu 3 (BTC) 2 ] n , BTC 3− = benzene-1,3,5-tricarboxylate) 21 and MOF-5 (Zn 4 O(BDC) 3 , BDC 2− = 1,4-benzodicarboxylate). 22 The reason for such reduced separation performance under humid conditions is the collapse of the framework by slow hydrolysis when exposed to moisture conditions, even to limited amounts of water. On the other hand, there are other MOFs built by a robust metal cluster that are water-stable, but a common decrease of CO 2 capacity and selectivity is also observed under humid conditions. 23−26 For instance, CO 2 adsorption capacity on UiO-66-NH 2 decreased 88% at 70% RH compared to that at dry conditions. 10 Considering that flue gas contains 5−7% water, these MOFs can only be used for CO 2 separation after a dehydration pretreatment of the gas stream, certainly increasing the economic costs and energy demand of the separation process. Consequently, new approaches are being investigated to increase the robustness of these MOFs absorbents and maintain their water stability, such as alkylamine grafting 27 or PEI composite impregnation. 28 In addition, functionalizing the original linker using hydrophobic functional groups such as fluorine or alkyl groups offers an opportunity to improve the hydrolytic stabilities and repel competitive water molecules from entering into MOFs. However, this methodology requires a more complicated organic synthesis reaction for obtaining the functionalized organic linker. 29−31 Given that water vapor is inevitable among most industrial flue gas mixtures, the screening of facile synthesizable water-stable MOFs for efficient CO 2 separation under humid conditions is both essential and challenging.
Icosahedral carboranes 1,n-C 2 B 10 H 12 (n = 2 (ortho or o), 7 (meta or m), or 12 (para or p)) are a class of commercially available and exceptionally stable 3D-aromatic boron-rich clusters that possess material-favorable properties such as thermal and chemical stability and high hydrophobicity. 32−37 Carborane-based linkers in MOFs were first reported in 2007 at Northwestern University. 38 The same group developed a family of thermally robust p-carborane-based MOFs over the following years, 39−45 some of those showing high gas sorption capacities and good sieving behavior for gas mixtures. However, the use of the highly expensive and more symmetric p-carborane as a scaffold for ligand synthesis prevents any further research on potential real-world applications. 46 Since 2016, some of us have pioneered the use of the cheaper o-47, 48 and m-carborane 49−55 derivatives with the primary objective of increasing the water stability of the prepared MOFs. We have demonstrated that introducing carborane moieties into MOFs can greatly enhance the framework's water stability. The high hydrophobicity of some of these MOFs has provided them with high hydrolytic stabilities that allow their use in applications where water is always present. This is the case of the m-carborane-based [Cu 2 (mCB-L) 2 (DABCO) 0.5 (H 2 O)] (mCB-MOF-1; mCB-L = 1,7-di(4-carboxyphenyl)-1,7-dicarbacloso-dodecaborane; DABCO = 1,4-diazabicyclo[2.2.2]octane; Figure 1), which remains intact in water solutions under very harsh conditions. 52 In this work, we have investigated our hydrophobic ultramicroporous meta-carborane-based MOF (mCB-MOF-1) for gas adsorption and CO 2 separation applications. We show that mCB-MOF-1 exhibits unaffected performance for CO 2 :N 2 separation under various humid conditions, validating the results by breakthrough dynamic separation experiments. Regeneration is also successfully achieved in mild conditions despite the presence of water, making this MOF a potential candidate for efficient CO 2 separation from flue gas in industrial applications. The performance of our material is also compared with the well-known water-stable MOF-74(Ni) and shows that mCB-MOF-1 is a superior adsorbent for the separation of CO 2 :N 2 mixtures under humid conditions. highly hydrophobic carborane moieties decorate the 1-D square MOF channels in mCB-MOF-1, thus providing protection to the Cu 2 paddle wheel units against hydrolysis or ligand displacement. Guest-free mCB-MOF-1, denoted mCB-MOF-1′, can be generated by heating at 120°C in a vacuum and thus provide unsaturated Cu sites. As previously established, mCB-MOF-1′ shows negligible water sorption and has a Brunauer−Emmett−Teller (BET) surface area and pore volume of 756 m 2 g −1 and 0.31 cm 3 /g, respectively, and it is also porous to CO 2 (1.34 mmol g −1 at 313 K and 2 bar). It is stable in air for at least two years and submerged in 90°C water for over two months.
Although high hydrolytic stability is a prerequisite for applications where water is present, effective gas separation and good selectivities are not granted. Thus, we first evaluated the ability of mCB-MOF-1′ to separate CO 2 from N 2 by single-gas isotherms measurements. Thus, CO 2 and N 2 sorption data at different temperatures and low pressure were collected ( Figure  2a,b). At 1 bar, the CO 2 uptakes on mCB-MOF-1′ were 2.15, 1.35, 0.97, and 0.80 mmol/g at 273, 298, 303, and 313 K, respectively, these values being compatible with those of other ultra-microporous MOFs. When compared with N 2 adsorption, the CO 2 uptake was much higher at 273 K, as shown in Figure 2c, with the adsorption amount of 2.15 mmol/g for CO 2 and 0.21 mmol/g for N 2 at 1 bar at the same temperature. These results demonstrate that CO 2 molecules have higher affinity with mCB-MOF-1′ compared to N 2 , highlighting its advantage for highly effective separation of CO 2 from N 2 .
To evaluate the evidenced adsorption affinity of the adsorbate−adsorbent, the isosteric heats of adsorption (Q st ) for both gases were derived from the static isotherms at different temperatures on the basis of the virial method (see details in Experimental Section and SI). As shown in Figure 2d, the Q st value of CO 2 was in the range of 24.7−25.3 kJ/mol, while the value of Q st for N2 was in the range of 8.2−8.5 kJ/ mol, indicating a much stronger interaction between CO 2 and mCB-MOF-1', compared to N 2 . The zero-coverage Q st for CO 2 was 24.7 kJ/mol, which is lower than that for the related Based on the adsorption performance difference for CO 2 and N 2 , an ideal adsorbed solution theory (IAST) was adopted to predict the adsorption selectivity of theoretical CO 2 :N 2 binary mixtures. The adsorption isotherms of CO 2 and N 2 were first fitted by dual-site Langmuir−Freundlich isotherm models (see details in Experimental Section and SI). The obtained fitting parameters are summarized in Table S1. It was noticed that both regression coefficients R 2 were higher than 0.9999, indicating the excellent fitting of the data. Figure 2c shows the IAST selectivities at 273 K for CO 2 :N 2 (with two different compositions: 0.05:0.95 and 0.2:0.8, v/v) in the pressure range 0−1 bar. Slightly lower CO 2 :N 2 adsorption selectivities at lower pressures were observed. At 1 bar, the CO 2 :N 2 adsorption selectivity at 273 K was in the range 25.5− 27.0, with maximum values of 26.8 and 26.9 for the CO 2 :N 2 ratios of 0.05:0.95 and 0.2:0.8, respectively. Nevertheless, there was no significant difference in the adsorption selectivities for the two ratios of gas mixtures. Overall, these results show that mCB-MOF-1' may exhibit good separation performance of CO 2 from N 2 .
Breakthrough measurements are commonly used to evaluate the potential of porous materials in gas separation processes in different conditions. CO 2 :N 2 gas mixtures were passed through a fixed-bed column that was filled with 200 mg (average) of mCB-MOF-1'. Breakthrough operation conditions ranged from 283 to 298 K, at 1 bar, in dry and humid conditions. The inlet mixture was set to a 15 mL min −1 flow of a dilution of CO 2 in N 2 (5, and 20%), and completed with an extra 1 mL min −1 of helium, used as a nonadsorbing trace. The use of a tracer is mandatory to assure the good performance of the measurement, setting time zero after it breaks through the column. In a typical experiment, the MOF sample was regenerated before the measurement at atmospheric temperature and pressure conditions of 15 mL min −1 Ar flow for 20 min. DrymCB-MOF-1 was activated at 393 K in a vacuum for 2 h. As moisture is a common pollutant in industrial gas flows, mCB-MOF-1 has been investigated under various conditions as shown in Figure 3.
We first conducted CO 2 :N 2 breakthrough experiments at two different CO 2 dilutions (5 and 20%) and two temperatures (283 and 298 K) using drymCB-MOF-1′ (Figures 3a, 4a,b, S1, and Table S2) and a mass spectrometer to measure the outlet gas concentration (Figure 4c). The breakthrough curves indicate that mCB-MOF-1′ could effectively separate the two distinct CO 2 :N 2 mixtures under ambient conditions. The N 2 breakthrough occurred first and subsequently reached a plateau early. In comparison, the CO 2 breakthrough time is significantly longer, confirming the effective separation performance of mCB-MOF-1′. The typical roll-up effect especially appears at a high concentration (20% CO 2 , Figures 4b and S1b). CO 2 selectivity is clearly observed also at the lower temperature of 283 K ( Figure S1), as it also takes longer for this gas to breakthrough the column. As thermodynamically expected, a significant increase in the amount of adsorbed CO 2 was observed at this temperature (Figure 4c and Table S2), a consequence of adsorption being an exothermic process. This remarkable selectivity (>1000; due to the negligible value of nitrogen adsorption), likely caused by the presence of open metal sites (OMS), makes mCB-MOF-1' a promising material in CO 2 :N 2 mixture separation. In addition, increasing the temperature, which favors the diffusion of the gases, causes a reduction in the CO 2 capacity (see Figure 4c), but the selectivity remains very high. Thus, it is clear that the MOF acts as a molecular sieve, avoiding N 2 adsorption and promoting CO 2 capture in gas streams.
The considerable CO 2 :N 2 separation performance combined with the hydrophobicity of mCB-MOF-1' prompted us to investigate the gas separation under high-humidity conditions. For that purpose, two different prehumidification steps were taken. (i) Wet-gas mixtures: CO 2 :N 2 mixtures were passed through a bubbler containing room-temperature DI water (Figure 3b), assuring that mCB-MOF-1′ was in permanent contact with a hydrated gas stream during the measurement. (ii) HydratedmCB-MOF-1′: the sample was prehydrated under a water-saturated atmosphere (100% humidity) for 24 h and before the breakthrough experiments (Figure 3c). Whereas conditions in (i) are close to industrial gas flows, those in (ii) will pose a real challenge for this water-stable adsorbate selectivity, as, in the case of mCB-MOF-1′, the active sites are open Cu sites with high affinity for water molecules. 58 Thus, we performed the dynamic breakthrough measurements under the same conditions as those for drymCB-MOF-1′ (5 and 20% CO 2 dilutions; 283 and 298 K). The results (Figures 5, 6, S2, and S3 and Table S2) show that there are no significant differences in the amount of absorbed CO 2 between the dry, wet-gas, or hydratedmCB-MOF-1′, evidencing the remarkable water resistance of the adsorbent and CO 2 :N 2 separation performance even under high humidity (100% RH). Water is excluded from the pores due to the high hydrophobicity of the MOF channels. This leaves the open Cu sites available for the selective sorption of CO 2 over N 2 .
Breakthrough curves and times of the hydratedmCB-MOF-1′ ( Figure 5) are nearly identical to those for dry mCB-MOF-1′ (Figure 4). These experimental results further demonstrate that water does not effectively enter the pores of our MOF nor affects the CO 2 :N 2 separation selectivities during the breakthrough experiments. Effective separation and good selectivities were also obtained when using wet-gas mixtures ( Figure   S3). During the separation time, the outlet concentration of N 2 is higher than 99%, indicating the outstanding separation for CO 2 :N 2 even under humid conditions. Meanwhile, the CO 2 longer break time confirms that the interaction between CO 2 and mCB-MOF-1' is stronger than that of N 2 . Additionally, the captured amount of CO 2 was of the same order ( Figure 6) when using wet-gas mixtures or hydratedmCB-MOF-1′ or drymCB-MOF-1′. Regeneration tests show that the separation performance was maintained after the adsorption−desorption  cycles ( Figure S4) and the crystalline phase of mCB-MOF-1′ was preserved after the separation process ( Figure S5). These experimental results demonstrate that mCB-MOF-1 is a promising porous absorbent for CO 2 separation over N 2 for industrial processes.
Other MOFs with unsaturated metal centers in their structures, such as the MOF-74 family, are recognized as good candidates for the efficient postcombustion CO 2 capture from water-containing flue gas generated from coal-fired power plants. 59,60 From this family of MOFs, MOF-74(Ni) is one of the most water-stable ones. 61,62 We have therefore selected MOF-74(Ni) as a reference material for a comparison study with our mCB-MOF-1′ and performed dynamic breakthrough experiments on dry and hydrated MOF-74(Ni) samples (Figure 3a,c, respectively). Although the MOF remains stable at these conditions, the results represented in Figure 7 clearly show the competitive adsorption of water in MOF-74(Ni). This resulted in a drastic decrease in the CO 2 adsorption capacity and thus in its separation efficiency in the presence of water. The same trend is observed at all tested gas compositions and temperatures.
Regarding the related DABCO pillared MOFs [M 2 (1,4bdc) 2 (DABCO)] (M = Ni, Zn), both materials show good CO 2 :N 2 selectivities, but they collapse in a humid environment. 63 Although these MOFs do not contain free coordination sites for water to readily interact with, they are not stable under >60% relative humidity at RT. Degradation seems to be related to water adsorption at defect sites in the MOFs. 64 Considering water one of the most common contaminants, the possibility of performance in its presence is a relevant parameter for implementation in the current separation industry.

■ CONCLUSIONS
This carborane-based material (mCB-MOF-1) presents not only competitive capacity and remarkable selectivity values for carbon dioxide adsorption in CO 2 :N 2 mixtures, but it also stands for its excellent water stability along the separation process. The presence of unsaturated open metal sites explains the higher selectivity for CO 2 sorption than that for N 2 . H 2 O is excluded from the pores due to the high hydrophobicity of our MOF.
After testing the adsorbent in different concentrations, temperatures, and humid conditions, two statements highlight the potential of this adsorbent: (i) the sieving effect derives in complete gas separation, achieving high CO 2 capture efficiency, but allowing complete regeneration at mild   conditions; and (ii) adsorption properties remain constant under moisture conditions, placing mCB-MOF-1 as an interesting alternative in separation processes involving an extreme humidity atmosphere. Given the mild affinity between gas and adsorbent mCB-MOF-1, the gas separation with higher CO 2 concentration at a lower temperature is specifically suitable for industrial purification processes over highly contaminated flows.
In addition, MOF-74(Ni) has been used as a reference due to its water stability properties; mCB-MOF-1 surpasses its performance and suppresses the rest of competitors in such challenging moisture operation conditions. ■ EXPERIMENTAL SECTION Materials. All chemicals were of reagent-grade quality. They were purchased from commercial sources and used as received. mCB-MOF-1 was synthesized as previously reported. 52 Crystals of mCB-MOF-1 were immersed in acetone and exchanged once a day for three consecutive days, then filtered, and dried in air. The latter were then activated by heating at 120°C under a dynamic vacuum for 2h.
Characterization and Methods. Gas sorption−desorption measurements were performed using IGA001 and ASAP2020 surface area analyzers. The sample was first degassed at 130°C for 12 h. Powder X-ray diffraction (PXRD) was recorded at room temperature on a Siemens D-5000 diffractometer with Cu Kα radiation (λ = 1.54056 Å, 45 kV, 35 mA, increment = 0.02°).
The isothermal parameters were well fitted by the double-site Langmuir−Freundlich (DSLF) method from the pure CO 2 adsorption isotherms at 273 K. Fitting parameters of these equations as well as the correlation coefficients (R 2 ) are listed in Table S1. Predicted selectivity for binary mixtures of CO 2 /N 2 was analyzed using IAST.
Ideal Adsorbed Solution Theory (IAST) selectivity. To investigate the separation efficiency of CO 2 :N 2 mixtures for mCB-MOF-1, the IAST method was used to predict the molar loadings at specific partial pressures using pure single-component isotherm fits.
For the adsorption isotherm of CO 2 and N 2 on mCB-MOF-1' at 273 K, it was fitted with a double-site Langmuir−Freundlich (DSLF) model Here, q sat,1 and q sat,2 are saturation uptake (mmol/g) for sites 1 and 2, respectively; b 1 and b 2 are the affinity coefficients of sites 1 and 2, respectively; and c 1 and c 2 are the parameters for the deviations of an ideal homogeneous surface, respectively. In the Ideal Adsorbed Solution Theory (IAST), two-gas adsorption selectivity could be calculated from single-component isotherm fitting parameters, defined as follows To evaluate the interactions between mCB-MOF-1 and these gas molecules (CO 2 and N 2 ), the isosteric heat of adsorption Q st was calculated. In detail, Q st was obtained by fitting adsorption isotherms of CO 2 at 273, 298, and 303 K and N 2 at 77 and 313 K with eq 1. Then, Q st was calculated by eq 2.
where p and N are the pressure (Torr) and the quantity adsorbed (mg/g), respectively; T is the temperature (K); a i and b j are empirical parameters, respectively; m and n are the number of coefficients required to give a good fit to the isotherms, respectively; and R is the ideal gas constant (J·K −1 ·mol −1 ). Breakthrough Separation Experiments. Dynamic breakthrough experiments were done on an ABR (HIDEN Isochema) instrument. It is an automated breakthrough analyzer based on a fixed-bed adsorption column. In a typical experiment, pressure, temperature, and inlet composition are set and controlled. To determine the adsorption dynamic behavior of gas mixtures, the outlet flow composition is analyzed by an integrated mass spectrometer (HPR-20 QIC). The column was filled with 224 mg of mCB-MOF-1. Before every measurement, the sample was regenerated at atmospheric temperature and pressure in 15 mL min −1 Ar flow for 20 min. Operation conditions ranged from 283 to 298 K at 1 bar. The inlet gases mixtures consisted of a 15 mL min −1 dilution of carbon dioxide in nitrogen (5−20% CO 2 in N 2 ). In all situations, gas mixtures resemble expected natural or industrial compositions. Time zero, in the analysis, is set with the first detection of helium due to its use as a tracer (1 mL min −1 of He in the feed flow). When using wet-gas mixtures, it was introduced as a moisture desiccator after the column to avoid any damage to the instrument or the mass spectrometer. Any effect derived from the introduction of these elements was corrected with a blank measurement.